1. Field of the Invention
In this invention a method to realize fluxless indium bumping is described. This method can be used for the packaging for MEMS, photoelectric chip, bio-chip and so on. It is an invention in the field of microelectronics packaging.
2. Prior Art
Solder is a must for microelectronics packaging. Normally it can help to realize the signal transmission and provide mechanical support in and between chips. For a normal packaging process, flux is also a must. For example, when using Sn solder to weld chips without flux, it will be very difficult to get a high quality interconnect due to the oxidized surface. Such oxidized surface will prevent alloy formation when welding because it has a very high melting point. In order to get a high-quality interconnection, flux must be used for remove the oxide layer. But for some application, such as the packaging of the photoelectric chip (with optical lens), MEMS and biological detection chip, flux is prohibited to avoid its pollution to chip. The pollution probably causes device performance degradation and even completely failure. So it is necessary to develop a kind of flux free welding process for such applications. Normally two ways can be used to realize such kind of welding. One is with special flux such as hydrogen plasma, which can provide a strong educability environment; the other is to coat a special thin layer on the solder surface which can prevent the solder being oxidized. The second way is described in this invention.
The most difficult point for this technology is how to realize flux free solder bumping. So far two research groups have some studies on this technology deduced from the published paper. First one is Chin C. Lee's group, who succeeded to realize two chips welding with Tin-based solder and without flux [Chin C. Lee, Ricky W. Chuang. Fluxless Non-Eutectic Joints Fabricated Using Gold-Tin Multilayer Composite. IEEE Transactions On Components And Packaging Technologies, 2003, 26(2):252-263]; the other is Jeong-Won Yoon's group, who realized Tin-based solder bumping without flux[Jeong-Won Yoon, Hyun-Suk Chun, Seung-Boo Jung. Reliability evaluation of Au-20Sn flip chip solder bump fabricated by sequential electroplating method with Sn and Au. Materials Science and Engineering A, 2008, 473:119-125.]. In this invention, a method is proposed to achieve flux free indium-based bumping. As it is known, indium is a kind of metal with many excellent properties, such as low melting point, excellent plasticity, excellent welding characteristics in low temperature and so on. These properties are better than that of Tin-based solder.